Tablets of pool chlorination compounds have been marketed for many years with varying degrees of success. One past attempt is that shown in U.S. Pat. No. 3,856,932 issued Dec. 24, 1974 to May which discloses a solid disk shaped tablet with a water impervious barrier around its outer edge so that the flat upper and lower exposed surfaces would hopefully dissolve at a relatively uniform rate. However, large sizes of the May tablet may tend to obstruct pool skimmels where many pool owners like to place such tablets in order to keep them away from swimmers and to have a relatively constant flow of water past the tablet. Large solid tablets of the May design would cause consumers worry about this possible blockage of skimmers. The slowness of feeding of the May tablet might require an unacceplably large diameter tablet in order to have sufficient surface area to achieve a desired feed rate. In addition the water barrier of May might tend to come off in use or, if it stays on, block the flowpath of water over the top of the tablet as it dissolves. Another past attempt was my own U.S. Pat. No. 4,546,503, issued Oct. 15, 1985 which also involved a 4" tablet, but with a central hole and a surrounding cup and special peripheral shape. However, the cup was large enough that it again presented a concern to many owners of pools who were afraid its size would constrict flow inappropriately in a skimmer. Thus a new approach to tablet design is needed to overcome the above problems while still allowing a large tablet which can last a relatively long time and dissolve at a relatively even rate.